A computer network is a collection of interconnected computing devices that can exchange data and share resources. In a packet-based network, the computing devices communicate data by dividing the data into small blocks called packets, which are individually routed across the network from a source device to a destination device. The destination device extracts the data from the packets and assembles the data into its original form. Dividing the data into packets enables the source device to reseed only those individual packets that may be lost during transmission.
Network routers maintain routing information that describes available routes through the network. Upon receiving a packet, a router examines information within the packet and forwards the packet in accordance with the routing information. In order to maintain an accurate representation of the network, routers exchange routing information in accordance with one or more routing protocols, such as an interior gateway protocol (IGP) or Border Gateway Protocol (BGP). Routing information imported by a router or statically configured by an operator is stored in routing tables of the router and may be subsequently advertised by the routing protocols to the router's neighbors.
Routers operate according to a routing policy framework that is configurable to permit the network operator to control the flows of routing information and packets. A routing policy for a router controls the flow of routing information between the routing protocols and the routing tables as well as between the routing tables and the forwarding table. Import routing policies control the routing information that is placed in the routing tables, and export routing policies control the routing information that is advertised from the routing tables.